In many wireless systems control-channel signalling is needed that indicates the particular time-frequency transmission resources to which the data is mapped, and the format/mode in which the data itself is transmitted. For UMTS LTE Rel-8 downlink communication, the relevant control channel that includes resource allocation and transmission format information is known as Physical Downlink Control Channel (PDCCH).
Each PDCCH is transmitted using one or more Control Channel Elements (CCEs), where each CCE corresponds to a plurality of sets of four physical resource elements. The number of CCEs used for transmission of a particular PDCCH is determined according to the channel conditions by the primary station, the ENodeB (or eNB) in LTE.
PDCCH transmissions from neighbouring or overlapping cells at the same carrier frequency may interfere with one another. This problem will become especially pronounced if the cells in question are synchronized or nearly synchronized with time-aligned transmissions. Indeed, in this case, the two transmissions of PDCCH may occur exactly at the same time and collides one with another. On the other hand, for some transmission modes, such as CoMP (Co-operative Multi-Point), synchronisation is deemed essential for satisfactory performance. Indeed, Co-operative Multi-Point transmissions are generally MIMO transmissions where antennas from a plurality of cells are used for a single transmission to users.
Ways of mitigating the PDCCH interference are therefore required.
In LTE Release 8 the main mechanism for mitigating the effect of interference on the PDCCH is randomization of the Resource Elements (REs) in the frequency domain (over the system bandwidth) and in the time domain (over the OFDM symbols reserved for control channels).
Existing methods for increasing robustness by randomisation in LTE include:                Increased sparseness of control channels. This can be done e.g. by expanding to 3 OFDM symbols, even if this is not required by PDCCH loading.        Power boosting the PDCCH transmissions. This is effective against PDSCH interference since PDSCH is not power boosted. However, it needs to be combined with “increased sparseness” to be highly efficient.        Increased aggregation level used for PDCCH. This would increase robustness for an individual PDCCH transmission, but would lead to higher interference, so is not likely to be very effective.        
However, none of these methods are effective enough, especially in case of fully aligned transmissions, like for a single base station which is serving two or three interfering cells.